In certain industrial activities, such as when using electric arc furnaces in the steel industry, or when refining copper, oxygen is used in batch mode, with significant variations in flow rate and at moderately high pressures (of the order of a few bars to about twenty bars). Various solutions are conventionally used to accommodate these changes in flow rate.
For example, EP-A-0,422,974 in the name of the Assignee describes a "seesaw" method intended for the production of oxygen gas at variable flow rate. The demanded oxygen is drawn from a reservoir, brought to the working pressure by pumping, and vaporized by condensation of a variable flow rate of air to be distilled.
In this known method, it is easy to show that, in order to keep the supply and withdrawal flow rates of the distillation and apparatus constant, it is necessary to vary the entering air flow rate in the same direction as the variations in the oxygen consumption. In the case when oxygen is produced under pressure, the air which is condensed in order to vaporize the liquid oxygen is boosted by an additional booster and, when the oxygen demand varies, both the boosted flow rate and the flow rate compressed by the main compressor need to be varied significantly.
In consequence, in this known method, the compressor and, where appropriate, the booster are overengineered significantly in comparison with the nominal oxygen flow rate to be produced. They also operate for most of the time at flow rates which are very different from their nominal flow rate, and therefore with impaired efficiency. To this is added the fact that the continuous presence of a reserve of the two liquids is needed in order for the seesaw to operate properly.
It has also been proposed for the gas to be produced to be stored in the form of gas in an auxiliary tank or "buffer", at a pressure greater than the production pressure. However, this solution is not satisfactory because it requires very large buffers to be installed in order to cater for very long-term consumption peaks. Furthermore, producing all of the gas at the pressure of the buffer is expensive in terms of energy.